A highly accurate servo control system is used to position the read/write head over the required track, but, even so, the head isn't always positioned to exactly the same radial position. It's self-evident how an MFM could be used to read data from a platter that can no longer rotate, but what isn't as obvious is how the technique lends itself to recovering data that's been overwritten – at least in theory.ĭata is written to a hard disk's platter in concentric circles known as tracks.

This is measured using optical techniques.

As the tip moves over the surface of the object being imaged, the magnetic field of that object exerts a force on the tip, which in turn causes a displacement of the cantilever. However, an MFM is able to read a static magnetic field – so the platter doesn't have to be able to spin.Īn MFM is a laboratory instrument that has an ultra-fine magnetised tip suspended on a cantilever. It's claimed that this method offers an additional benefit – it can read data, overwritten or not, from the surface of a platter that is no longer able to spin because of damage.įor the normal read operation of a hard disk, the platter must be able to rotate because an electrical signal can only be produced in the read head's coil if it's moving with respect to the magnetic field.

The second theoretical technique for recovering data is to use a Magnetic Force Microscope (MFM). Whether it becomes feasible when national security is at risk is another question entirely, and one we're not likely to get an answer to from the Secret Service. The snag is that this would be hugely time-consuming and costly – which is why nobody offers such a service. By reading each bit thousands if not millions of times and averaging all those signals, electrical noise, being random in nature, would average out to zero – whereas the genuine signal would build up and become visible. However, Western Digital's Gerardo Bertero – while questioning whether the technique is really a practical proposition – did suggest one possible solution. The problem is that the signal to the previously written data is so small that it effectively gets lost in the random electrical fluctuations commonly referred to as noise. This method of extracting overwritten data is commonly reported and academic papers have been written on the subject, but we've been unable to find any organisation that claims to have done it successfully.

Recovering overwritten data